The invention relates to a method for producing composite elements according to the preamble of the main claim and a composite element corresponding to the preamble of the independent claim.
Composite elements in a sandwich arrangement with at least one core and cover layers are known. The core material is thereby built up based on rigid foam, semi-rigid foam or flexible foam polyurethane and/or polypropylene and/or expandable polystyrene (EPS) and/or styrene derivatives (maleic acid-anhydride-styrene-acrylic). In the case of thermoplastic structures, cover layers made of compact films are bonded by the foamed core to form the sandwich element and, after heating the composite, are formed in forming tools so as to form three-dimensional bodies, such as for example ready-made inside roof linings in automobile construction. The sandwich arrangement comprising core layers and cover layers is produced either by multilayer extrusion or by lamination. The decorative cover layers are generally not fibre-reinforced but can also be produced by applying fibres of different types (natural fibres, glass fibres) in the presence of thermoplastic binders, generally in the form of powders by means of sintering.
When producing polyurethane sandwich structures by cold-forming, a foamed core made of rigid or semi-rigid foam polyurethane, which is thermally formable still to a certain degree, is provided on both sides with cover layers made of glass fibre/thermoplastic. The cover layers are thereby applied almost exclusively today in the on-line method by sprinkling thermoplastic powders with simultaneous addition of cut glass fibres from a roving by means of a cutting tool onto a conveyor belt for the lower cover layer and likewise onto the foamed core plate for the upper cover layer. This composite is subsequently sintered in a continuous heating device, for example in a double belt heating press, bonded and subsequently formed directly in a forming tool, into which simultaneously a textile is also introduced as surface decoration, so as to form three-dimensional bodies, for example moulded inside roof linings for automobiles. Subsequently, trimming takes place by means of punching, water jet cutting or methods of this type and the finished product is obtained, which is suitable for sheeting. This product is suitable for normal temperature usage up to approximately 105° C.
In another embodiment of the polyurethane application, the high adhesive force of polyurethane mixtures is used for bonding with the cover layers. A mixture of polyurethane polyol components with activators and MDI (44′ methylene di(phenylisocyanate) is thereby sprayed on a carrier material, for example polyethylene film, paper with polyethylene and/or polyurethane film covering, and glass fibre pieces are applied to the lower and upper cover layers. The decorative covering is undertaken generally by a textile with a sealing film and the prepared composite is immediately formed in a forming tool preheated to approximately 50-80° C. and cured. The products obtained after trimming are also usable at increased temperatures up to approximately 125° C.
The disadvantage of all these known composites is the inadequate acoustic effect since either only closed-pore core materials are used or the cover layers are no longer permeable due to sealing measures as in the just described embodiments. As a result, the absorbent effect desired in the interior of for example automobiles for reducing airborne noise is no longer offered.
For acoustic insulation parts in the engine compartment, such as for example bonnets, there are used nowadays so-called light foams, polyurethane semi-rigid foams with low relative density of 6-20 kg/m3 and surface mats based on polyethylene terephthalate (PET)/cellulose/polyacrylic (PA)/C-polyacrylonitrile (C-PAN) and mixtures of these. The surface mats are provided with adhesive coatings based on polyethylene (PE) (CoPES/MF (chemical fibres made of polyester-copolymers/melamine formaldehyde) and are processed in hot tools at 140-200° C. to form formed parts. To date more rigid products have been produced for use mainly by choice of surface mats. The disadvantage is an expensive cover mat with reduced acoustic behaviour. If surface mats with inserts made of glass fibres, which were produced by extrusion coating with polypropylene, are used the acoustic properties are reduced likewise. In addition structures of this type are not stable enough at high temperatures greater than 100° C.